1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of Related Art
An image forming apparatus using an electrophotographic scheme regularly forms a patch image and adjusts image quality in accordance with the measurement of its density to maintain a constant image quality. Basically, time for such image quality adjustment is specially set aside. A patch image is sometimes formed in an intermediate area between front and back sides of paper to obtain a stable image quality during image formation.
In the case in which images are formed on both sides of paper, the paper shrinks due to fixing processing after the image formation on its front side. An image forming apparatus thus changes the reduction/magnification ratio of the image on the back side to correct the difference in position and size between the images on the front and back sides.
Examples of the methods to change the reduction/magnification ratio of an image include changing the rotation speed of a polygon mirror which changes the direction of laser light at the area between the front and back sides of paper.
There is a case in which the rotation speed of a polygon mirror is changed at an intermediate area and in which a patch image is formed in the same intermediate area for adjustment of image quality. A patch image, however, is usually formed after the change of the rotation speed of the polygon mirror because forming a patch image in parallel with the speed change of the polygon mirror causes deformation and variation in density of the patch image (see, for example, Japanese Unexamined Patent Application Publication Nos. 2011-100035 and 2007-334303). Although some patch images can be formed in parallel with the speed change, such patch images are limited to those which allow any positions, shapes, or densities, such as a patch image for toner consumption shown in Japanese Unexamined Patent Application Publication No. 2007-334303.
To achieve speed-up and improve productivity, however, an image forming apparatus is required to have a shorter interval on which continuous image formation is performed, i.e., a shorter intermediate area.
Under such circumstances, the techniques of Japanese Unexamined Patent Application Publication Nos. 2011-100035 and 2007-334303 change the rotation speed of the polygon mirror to a target speed at once and thus can shorten the time required for the speed change. Unfortunately, however, changing the speed at once in such a way causes unstable rotation immediately after the speed change. The image forming apparatus, therefore, has to wait to perform image formation until the rotation stabilizes. The remaining time after the rotation has stabilized is not enough for the formation of a patch image. It is difficult for such an apparatus to form a patch image in an intermediate area in parallel with changing the rotation speed of the polygon mirror at the same intermediate area.